The present invention relates to a constant-velocity joint through which driving and driven shafts are coupled together so that torque is transmitted from the driving shaft to the driven shaft, and an image forming device using such a constant-velocity joint, such as a copier or a laser beam printer (LBP).
A known image forming device comprises an image-forming unit or units including a photoconductor drum rotated in one direction, and an electrifier, an exposure unit, a developing unit and a transfer/electrifier that are arranged around the radially outer surface of the photoconductor drum. The electrifier uniformly electrostatically charges the entire radially outer surface of the drum. The light exposure unit then emits image-forming light based on image information onto the drum to form an electrostatic latent image on the drum. The developing unit supplies toner onto the latent image to form a toner image. The toner image is transferred onto a transfer material that is being fed by the transfer/electrifier at the same speed as the peripheral speed of the photoconductor drum to form an image on the transfer material.
A monochrome image forming device comprises a single such image forming unit. A full-color image forming device comprises four such image forming units arranged in tandem in the direction of feed of the transfer material, the respective image forming units forming yellow, magenta, cyan and black toner images, which are transferred onto the transfer material to form a color image.
In either of the monochrome and full-color image forming devices, if the photoconductor drum is not rotated at a constant speed per rotation, the electrostatic latent image formed on the photoconductor drum by the exposure unit will partially shrink and partially expand, thus causing the toner image to partially shrink and partially expand, too, when transferred onto the transfer material. Thus, the ultimately obtained image tends to be warped and low in quality.
For the above reason, the photoconductor drum has to be always rotated at a constant speed.
In one conventional image forming device, the rotary shaft of a motor is directly connected to the shaft of the photoconductor drum to rotate the photoconductor drum with the motor. But since it is extremely difficult to support the drum and mount the motor such that the shaft of the drum and the rotary shaft of the motor exactly align with each other, and instead, the drum shaft and the motor rotary shaft tend to incline relative to each other or otherwise get out of alignment with each other, the photoconductor drum cannot usually be rotated at a constant speed. The image ultimately obtained thus tends to be low in quality.
In order to solve this problem, in an image forming device disclosed in JP patent publication 5-341589, the shaft of the photoconductor drum is coupled to the rotary shaft of the motor through a joint, and the position of the drum at which the latent image is formed by the exposure unit is arranged diametrically opposite to the position at which the toner image is transferred onto the transfer material. With this arrangement, although the photoconductor drum cannot be rotated at a constant speed because the joint used here is a nonconstant-velocity joint, it is expected that any shrinkage and expansion of the latent image due to uneven rotational speed of the drum can be canceled by shrinkage and expansion of the toner image when transferred onto the transfer material, so that the image transferred onto the transfer material is free from any shrinkage and expansion.
But actually, it is difficult to arrange the above two positions exactly diametrically opposite to each other due to assembling errors and/or inclination of the photoconductor drum. Thus, it will be difficult to completely cancel any shrinkage and expansion of the latent image with the shrinkage and expansion of the toner image. The ultimately obtained image thus tends to be low in quality.
On the other hand, there are known slide type constant-velocity joints that permit angular and axial displacements of the two shafts. A typical such constant velocity joint is disclosed in JP patent publication 52-34699 which comprises an outer ring, and an inner member inserted in the outer ring around the axis of the outer ring to define an annular gap there between. Three axial track grooves are formed in at least one of the radially inner and outer walls defining the annular gap so as to be circumferentially spaced apart from each other by 120 degrees. The joint further includes a cage inserted in the annular gap and formed with three pockets radially opposite to the respective track grooves. A ball is received in each pocket of the cage so as to roll along the corresponding track groove so that torque is transmitted between the outer ring and the cage through the balls.
The constant-velocity joint disclosed in this publication has its outer ring, inner member, cage and balls all formed of a metal such as steel. Thus, while the joint disclosed in this publication is high in strength, it is heavy, needs grease lubrication, and tends to produce large operating noise. Its use is therefore limited. For example, it is not suitable for use in office machines, audio devices, medical instruments, household appliances, etc.
If such a constant-velocity joint is used in an image-forming device, grease may leak, thereby soiling the transfer material. Thus, such a joint needs some measures against leakage of grease such as a boot. This necessarily increases the number of parts and thus the cost, as well as the size, of the constant-velocity joint.
An object of the present invention is to provide a constant-velocity joint which needs no grease lubrication, is lightweight and small in size, produces less noise, and can be used practically under any circumstances, and to provide an image forming device which can produce high-quality images by always rotating the photoconductor drum at a constant speed.